Auto-torque, tamper-proof screws

ABSTRACT

A screw having a screw-threaded shank and a circular head, the head being provided with three radially extending screwdriver tip receiving portions defined between three first, off central integrally formed projections designed to become shaved-off the screw head by a pre-set force applied during tightening of the screw, and three second, integrally formed projections designed to withstand a shear force greater than the pre-set force applied during slackening of the screw.

The present patent application is a non-provisional application ofInternational Application No. PCT/IL2003/000611, filed Jul. 24, 2003.

FIELD OF THE INVENTION

The invention relates to screws, and more specifically to restrictedtorque fastening screws.

BACKGROUND OF THE INVENTION

In the relevant industries, it is often required that fastening screwsbe tightened by a limited, controlled force, in order to avoid damage tothe equipment or to fulfill other conditions, e.g. in case of a coverwith a rubber seal which must not become over-squeezed.

Other, related examples, are cases where it is requested that oncetightened, the screw cannot be slackened by ordinary tools such as ascrewdriver, or at all.

Alternatively, a demand may be posed that it would be possible tounscrew the screw, but there will be clear and unmistakable indicationthat the screw has been tampered with by an unauthorized person.

The invention aims at proposing a solution satisfying any and all ofthese requisites.

It is a further object of the invention that the screw heads be producedby forging.

SUMMARY OF THE INVENTION

Thus provided according to the invention is a screw having ascrew-threaded shank and a circular head, the head being provided withat least one radially extending screwdriver tip receiving portiondefined between a first off-central, integrally formed projectiondesigned to become shaved-off the screw head by a pre-set shear forceapplied during tightening of the screw, and a second, integrally formedprojection designed to withstand a shear force greater than the saidpre-set force applied during slackening of the screw.

There are preferably provided three pairs of the first and the secondprojections equi-angularly located on the head of the screw wherein thefirst projections are of a generally right-angled triangularcross-section seen in a plane normal to the radial direction of thescrew head, and the second protrusions are of a generally isoscelestriangle cross-section seen in a plane normal to the axis of the screwhead.

A tool is provided for operating screws according to the invention,comprising a circular handle and three, equi-angularly arranged andradially extending ribs fitting into the said screwdriver tip receivingportions.

BRIEF DESCRIPTION OF THE DRAWINGS

These and additional constructional features and advantages of theinvention will become more clearly understood in the light of theensuing description of few preferred embodiments thereof, given by wayof example only with reference to the accompanying drawings, wherein

FIG. 1 is a three-dimensional view of a screw according to a firstpreferred embodiment of the invention;

FIG. 2 is a top view of the screw of FIG. 1;

FIG. 3 is a section taken along line III—III of FIG. 2;

FIG. 4 is a side view of the screw of FIG. 1;

FIG. 5 illustrates a tool for tightening the screw of FIG. 1

FIG. 6 is a three dimensional view of a screw according to a secondpreferred embodiment of the invention;

FIG. 7 is a top view of the screw of FIG. 6;

FIG. 8 is a side view of the screw of FIG. 6;

FIG. 9 is a three-dimensional view of a screw according to a thirdpreferred embodiment of the invention;

FIG. 10 is a top view of the screw of FIG. 9;

FIG. 11 is a side view of the screw of FIG. 9;

FIG. 12 is a partial cross-sectional view taken along line XII—XII ofFIG. 10;

FIG. 13 a schematically shows the tightening operation of the screw ofFIG. 9 by a designated tool;

FIG. 13 b shows the unscrewing operation of the screw using the sametool;

FIG. 14 is a three dimensional view of a screw according to a fourthpreferred embodiment of the invention;

FIG. 15 is a top view of the screw of FIG. 14;

FIG. 16 is a side view of the screw of FIG. 14;

FIG. 17 a shows the tightening operation of the screw at FIG. 14 by adesignated tool; and

FIG. 17 b shows the unscrewing of the same screw.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The embodiment of FIGS. 1–4 relates to a screw with limited tighteningtorque and indication if opened without authorization. The indication isgiven in such a manner that if the screw has been unscrewed—it cannot beclosed again whatsoever.

Hence, the screw generally denoted 10 comprises a shoulder(machine-screw) head 12 and a shank 14. As will be made clear, the screwcan be of any kind, either of the standard or of special design.

A pattern of cavities or depressions is applied to the top of the head12, most conveniently, and as normally applied in the production ofbolts and screws, by forging, in a one-time operation, as will be nowdescribed with respect to the present example. Variations andmodifications of the pattern will be readily apprehended afterunderstanding the design principles governing this embodiment.

At least one—but preferably three—radially extending slots 16, 18, 20are present, equi-angularly arranged as shown. The slot 16 (see FIG. 3)is delimited at one side thereof by a first, off-centrally locatedprojection 16 a of a generally right-angled triangular cross-section(seen in a plane parallel to the axis of the head 12). The surface ofthe hypotenuse 16 b slopes from the root of the projection 16 a up tothe original top level of the head 12 (projection 18 c—see below).

At the other side of the slot 16 there is formed a solid projection 16 cof a generally isosceles triangle cross-section as seen in a planenormal to the axis of the head 12.

Similarly, slot 18 has triangular projection 18 a and curved surface 18b at one side, and projection 18 c at the other side; and the sameapplies to the slot 20. Preferably, a circular depression (or boss) 22is made at the center (the apexes of the triangles 16 c, 18 c, and 20c).

An operating tool or driver for the screw 10 is shown in FIG. 5. Thetool denoted 24 has a hand gripping handle (or electric screwdriverstandard coupling tip) 26, three elongated radially extending ribs 26 a,26 b, 26 c and a central pin 28, designed to fit the slots 16, 18, 20,and the depression (or boss) 22, respectively.

In use, namely fastening a workpiece by the screw 10, the screw isturned by the tool 24 and tightened by an increased torque until theprojections 16 a, 18 a, and 20 a are simultaneously torn or shaved offby the shear force applied to the respective roots thereof.

The amount of the maximum applicable torque can be pre-determined bycorrectly calculating the collective shear strength that will cause thebreaking of the projections, the relevant parameters being thecross-sectional area of the roots and the shear strength of the materialthe screw is made of.

Once broken, further rotation of the tool would simply “throw” the toolout of the respective slots to merely idle over the top of the screwhead 12.

However, and in that respect uniquely different from prior artauto-torque screws, although opening of the screw 10 remains possible,by turning the head 12 in the opposite (counter-clockwise) direction bythe tool 24, the re-fastening thereof is not possible.

Hence, let us take the following frequent example where a manufacturerof certain equipment makes it a condition to the validity of itswarranty that no one except authorized personnel would be allowed todismantle any part of it. Once a purchaser attempts to release thescrews by himself, he will be compelled to replace them by “ordinary”screws when bringing the equipment to repair under the warranty, whichwill immediately indicate that the above condition has been violated andfree the manufacturer of its obligation.

FIGS. 6–8 exemplify application of the invention to a flat head (wood)screw and is otherwise analogous to the preceding embodiment andtherefore similar reference numerals are used.

Hence, provided at the screw 110 are three slots 116, 118, 120 and theirrelated shearable projections 116 a–120 a, etc.

In the modified embodiment of FIGS. 9–13, no screwdriver slots in thenormal sense of the term are present, but rather two sets ofunidirectional projections are formed as will now be described. Machinescrew 210 with shoulder head 212 is formed with a first set ofprojections—three in the described example—denoted 230, 232, and 234which are functional for closing the screws; a second set of projections240, 242, 244 for unscrewing; and central bore (or projection, at theoption of the designer) 250.

Again emphasized is that this pattern is easily attained bypress-forging so that, as in the preceding embodiments, the projectionsare integrally formed with the head 212.

The first set of projections 230–234 have a right-angled trianglecross-section. The hypotenuses of the triangles extend radially facingthe same, clockwise direction.

The second set of projections 240–244 are spaced from the projections ofthe first set both in radial and in the angular directions, withhypotenuses facing the opposite, counter-clockwise direction.

The total roots area of the first set is calculated to yield (becomeshaved off) by shear, under a pre-determined force applied by a drivingtool (see FIG. 5) represented by the shadowed surface depicted in FIG.13 a.

The total roots area of the second projections would preferably be madeto withstand a greater force, for unscrewing (in the appropriatecases—if at all) by turning the screw-driving tool in thecounter-clockwise direction as depicted in FIG. 13 b.

Once torn away by the shear force applied during the tightening stage(FIG. 13 a), the second set of projections neutralizes the operation ofthe tool in the closing direction due to the sloping sides of theprojections 240–244.

The embodiment of FIGS. 14–17 is a modification of the precedingembodiment. As evident, the shear-controlled projections are thosedenoted 340, 342, and 344, namely located around the outer circumferenceof the head 310, whereas the unscrewing projection 330–334 are deployedalong an inner circle.

Also noteworthy is the fact that in this example the driving tool (seenas shadowed areas in FIGS. 17 a and 17 b) is not identical for closingand for opening the screw (one being a mirror-image of the other).

The invention thus offers a novel and efficient solution fulfilling theobjectives as specified in the preamble paragraphs above.

Those skilled in the art to which this invention pertains will readilyappreciate that numerous changes, variations and modifications can beeffectuated without departing from the true spirit and scope of theinvention as defined in and by the appended claims.

1. A screw having a screw-threaded shank and a circular head, the headbeing provided with at least one radially extending screwdriver tipreceiving portion defined between a first integrally formed projectionof a generally right-angled triangle cross-section seen in a planeparallel to the axis of the screw head wherein the hypotenuse of theright angled triangle slopes from the bottom of the first projection tothe top thereof such that it is designed to become shaved-off the screwhead by a pre-set shear force applied during tightening of the screw,and a second integrally formed projection designed to withstand a shearforce greater than the said pre-set force applied during slackening ofthe screw.
 2. The screw of claim 1 comprising three pairs of the firstand the second projections equi-angularly located on the head of thescrew.
 3. The screw of claim 2 wherein the sloping side of the firstprojections face the tightening, clockwise direction of the screw. 4.The screw of claim 3 wherein the second projections are of a generallyisosceles triangle cross-section seen in a plane normal to the axis ofthe screw head.
 5. The screw of claim 3 wherein the second projectionsare of a generally right-angled cross-section seen in a plane parallelto the axis of the screw head.
 6. The screw of claim 5 wherein thehypotenuse of the right angled triangle slopes from the bottom of thesecond projections to the top thereof.
 7. The screw of claim 6 whereinthe sloping side of second projections face the slackening,counter-clockwise direction of the screw.
 8. The screw of claim 1further comprising a circular depression at the center of the head. 9.The screw of claim 1 further comprising a circular boss at the center ofthe head.